Writer's Block is something you have to micro-manage your sim into getting. Once they have it... and the negative moodlet wears off, the aspiration goal gets completed.
To get your writer to have writers block is very tricky. You need to bottom out your sims fun and probably social meters, add into the mix them being stressed if you can and basically have their food and potty needs totally topped up... then sit them at a computer and have them write non-stop for as long as it takes for them to get writer's block.
You'll know when they've got it as the game lets you know. There are explanations on the forum that have more detail... I just can't find the threads to point you to them. If your sim just stops writing... get them straight back to it! If they need to eat or potty... do it fast! Then back to writing, better to let them nearly starve etc because for every minute not-writing... you need to make that back up and that takes about three times as long. (two mins for potty = 6mins more writing to get back to where you were... kind of thing)
The higher skill level your sim has in writing the harder this goal is to do so stop them writing until you are ready to tackle this goal. It took one of my sims HOURS to get.. so you need to make sure it's not a work day. Queuing up a few genre books in a row is perhaps better then just using the interaction 'practice writing'.
Good luck, have a look around the forum for the info that is here... somewhere!
